Basically, most of highly efficient electronics are in a rigid and planar shape and use a single-crystal inorganic material such as silicon and gallium arsenide. On the other hand, when a flexible substrate is used, a flexing resistance is demanded for wirings. Further, in a use such as an electrode for actuator and transducer, and a skin sensor, it is demanded that the electrode and wiring can follow a deformation of a substrate, a dielectric film, etc. made of elastomer, etc. For example, in an actuator, the dielectric film expands and contracts depending upon changes in a voltage applied thereto. Therefore, it is necessary that the electrodes arranged on both sides of the dielectric film can expand and contract depending on the expansion and the contraction of the dielectric film so as not to prevent movements of the dielectric film. Moreover, in addition to the ability of expansion and contraction, it is demanded that changes in an electric resistance are small upon being expanded and contracted.
Further, many electric wires are used for a power supply and for a signal transmission in robots and wearable electronic equipments. In this regard, since the electric wire itself usually has almost no stretchability, it is necessary to arrange the electric wire giving allowance so that movements of robots and humans are not prevented. Such necessity causes a trouble in an actual use. Accordingly, there has been an increasing demand for a stretchable electric wire.
In a field of health care, there has been also a demand for an electroconductive material exhibiting a high stretchability. For example, by using a film made of a stretchable electroconductive material, it is now possible to develop a device which is capable of adapting to a human body being soft, flexible and curvilinear, via a close adhesion thereto. The use of the device as such covers not only a measurement of electrophysiological signal but also a delivery for advanced therapy and an interface between human and machine.
One of methods for solving problems in developing the stretchable electroconductive material is to use an organic electroconductive material. However, although the material which has been used up to now is flexible, it cannot be said to be stretchable. Accordingly, it cannot cover the curvilinear surface. Therefore, it is lacking in a reliability for its property and for its integration into a complex integrated circuit. Although a film made of other materials such as metal nanowire and carbon nanotube are promising to some extent, they are lacking in a reliability and are expensive. Accordingly, a development therefor is difficult.
An elongation rate necessary for the stretchable electroconductive film varies depending upon an actual use. In an expected use such as a stretchable wiring, stretchable antenna and stretchable electrode used in the field of FPC, robot, smart wear, health care sensor, display, solar battery, RFID, game machine, etc., it is desirable that a specific resistance is less than 1×10−3 Ωcm and also that an expansion is more than about 35%. In an electroconductive film formed by coating or printing of an electroconductive silver paste wherein silver powder is dispersed in resin, it is usual that the electroconductive film is broken or its specific resistance greatly increases upon expansion. The specific resistance upon expansion is desirable to be less than 1×10−2 Ωcm. When the specific resistance becomes higher, a circuit resistance value becomes high in such a use wherein a fine wiring or long wiring circuit is necessary. Accordingly, such an electroconductive film cannot be used.
Further, when an actual use is considered, it is desired that not only the stretchability is large but also changes in the specific resistance upon being subjected to the repetitive expansion and contraction are small. For example, when a wiring which is closely attached either to the human body directly or to a clothing worn by humans or when a wiring and a sensor which are for curved parts of robot are anticipated, they are repeatedly deformed by every movement in some of the sites and, as a result thereof, the wiring itself is also repeatedly expanded and contracted. Even under such a situation, it is still desirable that the changes in the specific resistance are small. Moreover, in a wiring and an electrode on a substrate material, an adhesion between the substrate material and the electroconductive film becomes small during the stage of being subjected to repetitive expansions and contractions whereby there is a possibility of resulting in a breaking of the wiring or the like. To be more specific, at least 20% of a repetitive stretchability has been demanded in a joint area of a human-type robot and a human body sensor.
In recent years, electric and electronic instruments are becoming lighter, thinner, shorter and smaller and becoming highly functional. As a result thereof, there has been a progress that a width and interval of connecting terminals become into fine pitches and into multiple wirings. Accordingly, an anti-migration property between wirings has been also demanded. Migration is a phenomenon that, when voltage is applied in a presence of moisture, silver powder is ionized, separated out, grown into dendrites and causes a short circuit between electrodes. Even in the case of the stretchable wiring, an excellent anti-migration has been also demanded in view of a reliability.
When distribution and selling are taken into consideration, the electroconductive silver paste is demanded to have a stability upon its storage for a long period. During the process from a production until a consumption, at least three months are demanded in view of a practical use. With regard to conditions for the storage, it is also desired in view of cost burden for a facility, utility, etc. that the paste can be stored in a refrigerator or, further, at room temperature (25° C.) rather than in a freezer.
As to approaches for developing a stretchable flexible wiring, two methods have been mainly reported.
One is a method wherein a corrugated structure is constructed so as to make even fragile materials stretchable (see Non-Patent Document 1). In this method, a metallic thin film is prepared on a silicone rubber by means of vapor deposition, metal plating, photoresist treatment, etc. Although a metallic thin film shows a stretchability of only a few percents, a stretchability can be shown when the metallic thin film is made in a zigzag shape, a continuous horseshoe shape or a corrugated shape, or when the metallic thin film is made in a wrinkled shape or the like by forming the metallic thin film on a previously stretched silicone rubber. However, in any of the above, an electric conductivity lowers to an extent of two digits or more when the metallic thin film is stretched by several tens percents. In addition, since the silicone rubber has a low surface energy, an adhesion between the wiring and the substrate is weak whereby there is a disadvantage that a metallic thin film easily peels off upon stretching. Accordingly, in this method, it is difficult to achieve both a high electrical conductivity and a high stretchability. Moreover, there is another problem that manufacturing costs are high.
Another approach is a composite material consisting of an electrically conductive material and an elastomer. Advantages of this material are an excellent printing property and stretchability. In a commercially available silver paste used for electrodes and wirings, a high amount of silver powder is filled in and compounded with a high elastic modulus binder resin. As a result thereof, a flexibility is poor and a modulus of elasticity is high. Upon stretching, cracks are generated and an electrical conductivity significantly lowers. In view of the above, the investigations have been carried out for a rubber and elastomer as a binder for imparting the flexibility. Also, the investigations have been carried out for a silver flake, a carbon nanotube, a metal nanowire, etc. which have a large aspect ratio as a conductive material and a high electrical conductivity for lowering the filling rate of a conductive material. In a combination of silver particles with a silicone rubber (see Patent Document 1), a decrease in an electric conductivity upon extension is suppressed by such a means that an electroconductive film on the silicone rubber substrate is further coated with a silicone rubber. In a combination of silver particles with a polyurethane emulsion (see Patent Document 2), although a high conductivity and a high elongation rate are reported, methods for dispersing the silver particles are limited because of an aqueous system whereby it is difficult to achieve an electroconductive film wherein silver particles are well dispersed. In addition, there has been no report for a repetitive stretchability. A combination of a carbon nanotube with an ionic liquid and vinylidene fluoride (see Patent Documents 3 and 4) has been also reported but an electric conductivity resulted thereby is too low and uses thereof are restricted. As such, it is a current status that a high electric conductivity and a high stretchability are hardly compatible with each other. On the other hand, there has been a report for a composite material which is printable, highly conductive and can be expanded and contracted by means of a combination of silver particles in a micron size with poly(vinylidene fluoride) and carbon nanotube wherein the surfaces are modified with self-organized silver nanoparticles (see Non-Patent Document 2). In this regard, although a high repetitive stretchability is reported therefor, the composite material is broken when an elongation rate reaches 35%. Also, since a carbon nanotube having a big aspect ratio is compounded, there is a possibility that an anisotropy in a conductivity and a mechanical property is resulted between a print direction and a right-angled direction thereto, when the composite material is made into a film by means of printing or the like. Accordingly, it is not preferred in view of the practical use. In addition, a surface modification of the carbon nanotube by the silver nanoparticles is complicated in its manufacture, resulting in high costs which are not preferred. As mentioned hereinabove, it is a current status that there has been almost no electroconductive silver paste which is capable of forming an electroconductive film such as a wiring, an electrode, an antenna and a sheet having a high elongation rate and a highly repetitive stretchability.